As a method for transmitting data with serial transmission, there is a well-known text broadcast system that transmits text broadcast data using a vertical blanking interval of a video signal.
In various regions across the world, text broadcast data that are superimposed upon a vertical blanking interval of a video signal are transmitted. FIG. 14 shows a table indicating the relationships among the types of text broadcast, the superimposition lines on which text broadcast data are superimposed, and the transmission clocks in the respective regions.
Conventionally, a text broadcast data extraction apparatus as shown in FIG. 13 has been employed to extract text broadcast data from a video signal including the text broadcast data superimposed thereon. Hereinafter, the conventional text broadcast data extraction apparatus will be described with reference to FIG. 13.
In FIG. 13, an analog video signal S1301 including text broadcast data superimposed upon a vertical blanking interval is inputted through a video signal input terminal 1301. An A/D converter 1302 converts the analog video signal S1301 into a digital video signal S1302. A low-pass filter (LPF) 1303 subjects the digital video signal S1302 to noise elimination. A sync separation circuit 1304 separates a horizontal synchronizing signal S1304a and a vertical synchronizing signal S1304b from the digital video signal S1302. A slice level calculation circuit 1305 calculates an optimum slice level S1305 for the signal S1303 that has been subjected to the noise elimination, on the basis of the horizontal synchronizing signal S1304a and the vertical synchronizing signal S1304b. A binarization circuit 1306 converts the signal S1303 that has subjected to the noise elimination, into a binary signal S1306 based on the slice level S1305. A PLL circuit 1307 generates an extraction clock S1307 that is synchronized with a clock run-in (CRI). A controller 1308 controls the PLL circuit 1307. An extraction circuit 1309 extracts the text broadcast data S1309 from the binary signal S1306, in accordance with the extraction clock S1307. A decoder circuit 1310 carries out a decoding process to the text broadcast data S1309. A decoded signal S1310 is outputted through an output terminal 1311.
The operation of the conventional text broadcast extraction apparatus that is constructed as described above will be described.
When an analog video signal S1301 including text broadcast data superimposed upon a vertical blanking interval is inputted through the video signal input terminal 1301, the analog video signal S1301 is sampled based on a sampling clock fs (MHz) by the A/D converter 1302, to be converted into a digital video signal S1302. The obtained digital video signal S1302 is inputted to the LPF 1303 and the sync separation circuit 1304.
The digital video signal S1302 inputted to the LPF 1303 is subjected to noise elimination, and outputted to the binarization circuit 1306.
On the other hand, a horizontal synchronizing signal (HSYNC) S1304a and a vertical synchronizing signal (VSYNC) S1304b are separated from the digital video signal S1302 inputted to the sync separation circuit 1304, and the separated signals S1304a and S1304b are inputted to the slice level calculation circuit 1305. Then, the slice level calculation circuit 1305 calculates an optimum slice level S1305 for text broadcast data in a predetermined line and a predetermined position of the digital video signal S1303 that has been subjected to the noise elimination, on the basis of the horizontal synchronizing signal S1304a and the vertical synchronizing signal S1304b, and outputs the obtained slice level S1305 to the binarization circuit 1306.
The binarization circuit 1306 converts the signal S1303 outputted from the LPF 1303 into a binary signal that is composed 0 and 1, based on the slice level S1305 calculated by the slice level calculation circuit 1305.
The PLL circuit 1307 establishes synchronization of an extraction clock having the same cycle as that of a transmission clock during a CRI period in the vertical blanking interval, for establishing synchronization between a text broadcast signal and the extraction clock, to generate an extraction clock S1307 that is synchronized with CRI. The extraction circuit 1309 extracts text broadcast data S1309 from the binary signal S1306 utilizing the extraction clock S1307, and the decoding circuit 1310 carries out a decoding process depending on the type of the text broadcast such as error correction. Then, the data S1310 decoded by the decoder circuit 1310 is inputted to a display circuit (not shown) through the output terminal 1311, and the display according to the type of the text broadcast is realized.
However, in the conventional text broadcast extraction apparatus, when the signal is out of phase at some point of the line due to group delay or the like at the extraction of text broadcast data, which is performed utilizing the extraction clock S1307 generated by the PLL circuit 1307 and synchronized with CRI, the text data gets out of phase, thereby causing an extraction error.
To suppress the group delay, a multi-tap filter, i.e., a filter for performing high-order waveform equalization is required, while this filter has such a large circuit scale that the whole circuit scale of the apparatus is adversely increased.